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Help needed with sample selection biases

We are searching for a graduate student to assist us on a very short assignment about sample selection biases and Heckman Probit models. The help is not needed for estimating the models, but instead for reviewing the scenarios where the use of such models is theoretically appropriate or otherwise. For instance, we are particularly interested in determining if Heck Probit type models could be applied in situations where the response variable had the don’t know/refused option, which has been used for the selection equation in some published research. We seek help in understanding the assumptions in the model that would permit or restrict the use of Heck Probit model in such circumstances.